Over the years, film extrusion equipment has been optimized for the rheology of high pressure-low density polyethylene (HP-LDPE). More recently low pressure-low density polyethylene (LP-LDPE) has been increasingly used which has different rheological properties than HP-LDPE resins. The different molecular architecture of low pressure-low density polyethylene (LP-LDPE) results in a film processing behavior which requires different extrusion parameters. By way of background, conventional extruders commonly used for HP-LDPE include an elongated barrel which may be heated or cooled at various locations along its length and a screw which extends longitudinally through the barrel. The screw has a helical land on its surface which cooperates with the cylindrical internal surface of the barrel to define an elongated helical channel. Although the pitch of the screw may vary along the length thereof, it is common at the present time to utilize screws of constant pitch wherein the pitch is "square", that is, where the distance between adjacent flights is equal to the diameter. The screw is rotated about its own axis to work the plastic material and feed it toward the outlet end of the barrel.
An extruder screw ordinarily has a plurality of sections which are of configuration specially suited to the attainment of particular functions. Examples are "feed" sections and "metering" sections, which are of basic importance and are present in nearly all extruders for handling thermoplastic polymers.
A typical extruder screw feed section extends beneath and forwardly from a feed opening where polymer in pellet or powder form is introduced into the extruder to be carried forward along the inside of the barrel by the feed section of the screw. In this section the channel depth of the screw is usually large enough to over-feed the solid polymer. This is a desirable effect because the over-feeding action serves to compact and pressurize the polymer particles and form a solid bed of advancing material.
The working of the material generates heat, and melting of the polymer proceeds as the material is moved along the feed section of the screw. Actually, most of the melting occurs near the barrel surface at the interface between a thin melt film and the solid bed of polymer. This general pattern persists until a substantial portion of the polymer reaches the melted state. After some 40 to 70 percent of the polymer has been melted, solid bed breakup usually occurs, and at this time particles of solid polymer become dispersed in the polymer melt. From this point on, it often is advantageous to intimately mix the polymer melt with the unmelted material to accelerate melting and minimize local non-uniformities.
An extruder screw "metering" section has as its special function the exertion of a pumping action on the molten polymer. Ordinarily the throughput achieved by a screw is thought of a being a function of the combination of the "drag flow" and "pressure flow" effects of the metering section.
Drag flow is basically the flow which results from the relative movement between the screw and the internal surface of the extruder barrel. It may be thought of as being proportional to the product of the average relative velocity and the channel cross-sectional area. This drag flow component is directed toward the outlet end of the screw. It may be increased by increasing the speed of the screw and/or by increasing the depth of the flow channel in the screw.
Acting in opposition to drag flow is a pressure flow component stemming from the reluctance of the material to flow through the restricted outlet opening at the end of the extruder passage. The speed of the screw does not directly affect the pressure flow component but, of course, it may effect such factors as back pressure and material viscosity, which factors, in turn, affect significantly the pressure flow component. On the other hand, pressure flow is directly affected by both the depth and length of the screw channel; an increase in channel depth has a tendency to increase greatly the pressure flow component and an increase in channel length has a tendency to reduce this back flow component.
In addition to the basic "feed" and "metering" sections an extruder screw also may include a number of other distinct sections. Nearly all screws include, for example, so-called "transition" sections.
Over the years, there has been a trend toward the use of extruders capable of high outputs. In many applications, various economies in production are possible where high extruder outputs can be obtained on a reliable basis.
Although LP-LDPE resins can be extruded on equipment designed for HP-LDPE resins, such as described above, certain equipment modifications are often required in order to extrude the low pressure resins at optimum conditions and at rates comparable to the high pressure resins. This is particularly true during extrusion of LP-LDPE which is subsequently processed into film. The problem appears to be that when the new low pressure resins are extruded through equipment with screws designed for the earlier high pressure resin pellets, they suffer from the effects of high exit temperatures, decreased energy efficiency and reduced outputs due to power limitations.
U.S. Pat. No. 4,329,313 issued to J. C. Miller et al on May 11, 1982 and which is assigned to a common assignee, discloses an apparatus and method for extruding ethylene polymers. According to the disclosure in said patent, which is incorporated herein by reference, an extruder screw is provided having at least three segments wherein the pitch remains constant in each segment and changes abruptly from one segment to the following segment and wherein the pitch ratio divided by the depth ratio is greater than 2/3.
The extruder screw disclosed in said patent is particularly suitable for extruding linear low density narrow molecular weight distribution ethylene polymers produced by low pressure processes which, as explained previously, have different rheological properties than low density polyethylenes produced by the so called high pressure processes.
The present invention provides an improvement in the apparatus and method disclosed in U.S. Pat. No. 4,329,313 which improvement permits operation at reduced temperature and increased output of the extruder screw.